A row has erupted in Australia after Sydney's zoo announced a nine-year-old Asian elephant was pregnant.

Animal rights groups say the elephant, Thong Dee, is too young to fall pregnant, and that the zoo is "irresponsible" in letting it happen.

This was "the equivalent of allowing your 12-year-old daughter to become pregnant", said Erica Martin of the International Fund for Animal Welfare.

But keepers at Taronga Zoo say they took expert advice and acted on this.

"We took our advice from the experts in elephant reproduction," says Lucy Melo, senior keeper at the zoo.

"And based on their findings, when they came here and did elephant reproductive assessments... they suggested that all of the elephants were reproductively viable, and that they actually recommended Thong Dee for immediate breeding."